In recent years a need has arisen for coating compositions that will function to replace chromates in metal treatment. This is due to the detrimental health and environmental impact that has been determined to be associated with chromium compounds.
Many chromate-free chemical conversion coatings for metal surfaces are known to the art. These are designed to render a metal surface "passive" (or less "reactive" in a corrosive environment), leaving the underlying metal protected from the environment. Coatings of this type that produce a corrosion resistant outer layer on the base metal or its oxide often simultaneously produce a surface with improved paint adhesion. Conversion coatings may be applied by a no-rinse process, in which the substrate surface is treated by dipping, spraying, or roll coating. The coatings may also be applied in one or more stages that are subsequently rinsed with water to remove undesirable contaminants.
Several metal and metaloid elements will form a continuous three-dimensional polymeric metal- or metaloid-oxide matrix from aqueous solutions. Chromium shares this characteristic along with silicon and other elements. The Group IV-A elements are attractive candidates for chromate replacement technologies as are the stannates as they share the virtue of being relatively innocuous environmentally and have common valences of +4, facilitating the formation of three dimensional amorphous coatings.
Chromate-free conversion coatings are generally based on chemical mixtures that in some fashion will react with the substrate surface and bind to it to form protective layers. The layer or layers may yield protection through galvanic effects or through simply providing a physical barrier to the surrounding environment.
Many of these conversion coatings have been based on Group IV-A metals such as titanium, zirconium and hafnium, a source of fluoride and a mineral acid for pH adjustment. The fluoride has heretofore generally been considered to be necessary to maintain the Group IV-A metal in solution as a complex fluoride. The fluoride may also serve to keep dissolved substrate metal ions (such as aluminum) in solution.
For example, U.S. Pat. No. 4,338,140 to Reghi discloses a coating for improved corrosion resistance with solutions containing zirconium, fluoride and tannin compounds at pH values from 1.5 to 3.5. Optionally, the coating may contain phosphate ions. U.S. Pat. No. 4,470,853 to Das is related to a coating composition comprised of zirconium, fluoride, tannin, phosphate, and zinc in the pH range of 2.3 to 2.95. According to Das, it is important that approximately 10 atomic percent of zirconium-zirconium oxide be present in the coating to obtain "TR-4" corrosion resistance. It was shown that coatings of higher zirconium oxide content produced excellent corrosion resistance. Compositions which gave higher zirconium oxide on the surface were preferred in the disclosures.
U.S. Pat. No. 4,462,842 to Uchiyama and U.S. Pat. No. 5,380,374 to Tomlinson disclose zirconium treatments in solutions containing fluorides which are followed by treatment with silicate solutions. This combination is suggested to form zirconate and syloxyl linkages (--O--Zr--O--Si--O--Si-- . . . ), yielding a coating with improved corrosion resistance over the zirconium treatment alone. Coatings of this type give excellent corrosion protection but very poor paint adhesion.
U.S. Pat. No. 4,863,706 to Wada discloses a process for producing sols and gels of zirconium and a process for producing zirconia. The processes described include reactions to produce basic boratozirconium and basic boratozirconium chloride sols. These are disclosed to be used in producing boratozirconium and boratozirconium chloride gels. A further object of the disclosure is to describe a method for producing zirconia from the gels at relatively low temperature. The essential components of the invention include a boron compound along with a polyvalent metal, zirconium and chloride.
U.S. Pat. No. 5,397,390 to Gorecki discloses an adhesion promoting rinse containing zirconium in combination with one or more organosilanes and fluoride. The compositions are used to rinse surfaces after they have been treated in a phosphating bath. The zirconium ion concentration is selected to maintain pH in a broad range as the silanes deposit on the substrate to promote paint adhesion and improve corrosion resistance. Organosilanes are necessary components of the disclosed compositions. Additionally, in preparing the compositions, Gorecki indicates that whenever zirconium-containing salts such as zirconium basic carbonate, zirconium hydroxychloride and zirconium oxychloride are used as a source (of zirconium) the salts must be dissolved in 50% hydrofluoric acid in order to effect dissolution. Gorecki does not indicate a necessity to dissolve the fluorozirconate salts mentioned in his disclosure. This demonstrates that fluoride is a necessary component of the disclosed compositions as it is included as part of the fluorozirconate salts or from hydrofluoric acid. Compositions of this nature are among the group of fluorozirconates which are referred to herein below as useful for "activating or activation" of a surface prior to application of the present invention.
Brit. Pat. 1,504,494 to Matsushima describes a process for treating metal surfaces using zirconium at a pH above 10.0. A zirconate coating is formed but the pH of the solution is maintained above the present invention.
It can be seen from the foregoing that the compositions of the prior art have not used Group IV-A metals in an aqueous, non-organic solvent containing systems that exclude fluoride specifically. Additionally, the prior art does not show formation and attachment of zirconate gels from aqueous solution without using organic solvents. Sol-gels are macromolecular units rather than discrete atoms or molecular units and are typically prepared from metal-alkoxy precursors in solvent-based solutions that are unstable in water.
The present invention employs an organic or inorganic oxyanion and certain nonoxy-anions to stabilize zirconium ions in an aqueous acidic solution with subsequent exposure of a metal substrate to the solution and with subsequent drying to produce a barrier of zirconium oxide coating. The prior art has demonstrated the usefulness of fluoride in compositions containing Group IV-A metals but has not shown the advantages of its exclusion from compositions containing these metals. Many health and environmental benefits of eliminating fluoride have been addressed in systems based on chemistries other than those of the Group IV-A metals. Examples are described in UK Pat. Application 2,084,614 by Higgins.
In the present invention, the zirconium (or other Group IV-A element) atoms are believed to bond to active oxygen atoms on the substrate surface, leading to a thin zirconate film forming from a reaction analogous to the reaction of silicates. Without rinsing the substrate before drying, the zirconate in the coating solution carried out with the substrate will bond to the thin film upon drying. Whereas silica "gels" form from alkaline solutions upon exposure to an acidic surface or one high in mono- and polyvalent cations, zircon "gels" will form on surfaces which are acidic or basic and those high in mono- and polyvalent cations. Upon drying at room or elevated temperature, a continuous polymeric zirconium oxide becomes fixed on the surface.
The present compositions and processes will give improved corrosion protection over zirconates containing fluoride in a ratios of greater than 2 fluoride atoms per zirconium atom. This is believed to be due to the fluoride competing with oxygen for bonding to zirconium in the matrix. With an atomic ratio of fluoride to zirconium at or between two to one and zero to one, the probability that all zirconium atoms will incorporate in the coating as a second or higher order oxide is very high. The term "order" is used here to describe the number of bonds a given Group IV-A element has to another element such as oxygen or fluorine; i.e. a second order zirconium fluoride has zirconium bonded to two fluorine atoms, a third order zirconium-oxygen compound has three zirconium to oxygen bonds, etc. With no fluoride present to compete with the oxygen, a three-dimensional zirconyl matrix with each zirconium atom bonded with up to four oxygen atoms will be established. Naturally occurring zirconates having this character are among the hardest, oldest and most stable inorganic compounds known. Studies by Connick and McVey (J. Am. Chem. Soc., Vol. 71, 1949, pp. 3182-3191) demonstrated that fluoride complexes of zirconium are far more stable than any other complexes (oxyanion and chloride) in their studies. It is this high stability of the fluocomplexes which interferes with Group IV-A oxide polymer formation. Its presence diminishes the Group IV-A to oxygen bond density (number per unit volume) and thereby decreases the protective ability of the metal oxide film. It is to be noted that Connick and McVey included chloride in the study and found its affinity to be on a par with the nitrate oxyanion. Thomas and Owens (J. Am. Chem. Soc. Vol. 57, 1935, pp.1825-1828) found nitrate and chloride anions to be comparable in many regards in their studies of zirconium hydrosols and developed a hierarchy for the tendency of anions to coordinate with zirconium. Again, fluoride was very high while nitrate and chloride were very low. The only anion stronger than fluoride was hydroxide. In the present invention, the formation of Group IV-A hydroxides is intended with eventual dehydration reactions leading to zirconyl-, titanyl- or hafnyl-oxide matrices.
With regard to nonoxy-anions (such as chloride) which may be suitable for stabilizing Group IV-A metals in aqueous solution yet still allow the formation of a titanyl, zirconyl or hafinyl matrix upon drying, the absolute value of charge to ionic radius ratio is the criterion for inclusion or exclusion in the group of preferred anions.
For example, a monatomic anion such as chloride with a charge of negative one and a radius of 1.81 Angstroms (According to Nebergall, Holtzclaw and Robinson, in: "General Chemistry," Publisher, D. C. Heath and Co., 1980) the value is .vertline.-1/1.81.vertline. or 0.552. For fluoride, the ratio is .vertline.-1/1.36.vertline. or 0.735. Therefore, it can be seen that when the ratio is below 0.735, the charge to radius (and therefore, overall atomic or molecular charge distribution) is such that the affinity will be lower than fluoride and acceptable for inclusion in the group of anions. An example of an anion excluded from the group would be sulfide with a charge of -2 and an ionic radius of 1.84 Angstrom units, resulting in a ratio of 1.087. Group IV-A sulfides are very stable and typically relatively insoluble as a result. This results in the exclusion of the S.sup.2- anion from the group of preferred nonoxy-anions.
In nonoxy-polyatomic anions, the radius may be considered to be the bond length between a central and periphery atom(s) (three or more atoms in the polyatomic anion) or simply the bond length in a diatomic anion. As with monatomic nonoxy-anions, the ratio of charge to radius determines the suitability for inclusion in the preferred group. Anions with an absolute ratio below 0.735 (charge to radius) are preferred.
The present invention may be used in processes where fluoride is used in preceding stages. This may cause accumulation of fluoride in the compositions of the present invention in some systems during processing. Fluoride may be tolerated in such cases up to a ratio not exceeding two fluoride atoms per Group IV-A atom in solution. It is to be understood that the presence of such fluoride is undesirable for compositions and processes described here but that such systems are still preferred to those with higher fluoride levels. In the prior art, fluoride is typically used at a ratio of at least four fluoride atoms per Group IV-A atom.
It should be further noted that the zirconate coatings containing fluoride are inferior to the same which are subsequently treated with silicate solutions. This indicates the silicate itself is superior to the fluorozirconates for protection and while the fluorozirconates give some benefit, they act primarily as a surface activator and attachment device for the silicate layers.
The present invention will provide improved, highly corrosion resistant conversion coatings based on Group IV-A metals such as zirconium by combining the Group IV-A metal with a stabilizing anion (oxyanions, haloanions and others) other than fluoride in acidic solution. The presence of fluoride in the solution is undesirable but may be tolerated up to a ratio of two fluoride atoms per Group IV-A atom.
In one aspect of the invention, the zirconium content of the solution is 1,000 to 20,000 ppm, 500 to 15,000 ppm nitrate and 1,000 to 7,000 ppm tris(hydroxymethyl)amino- methane; the preferred pH of the solution will be between about 1.0 and 4.0. The coating may optionally include Group IA and/or Group IIA elements, ethanol amines, organic acids such as acetic acid, sequestering agents, and chelants to inhibit precipitation caused by mono- and polyvalent metal ions that may build up in the coating solution.
One object of the invention is to provide improved Group IV-A conversion coatings for steel, magnesium and aluminum that are both highly corrosion resistant and simultaneously serve as an adhesion promoting paintbase. This is characteristic of chromate conversion coatings, but environmentally safe silicate coatings generally reduce paint adhesion.
An additional benefit of the invention is that the coating is formed from an aqueous solution with no organic solvents used. This eliminates the disposal and emission considerations involved in producing zirconates and other metal oxide-containing coatings from sol-gel applications, while providing a broad spectrum replacement for chromates.
Further objects and advantages of the present invention will be apparent from the following description and accompanying drawing.